Kinematic MRI imaging of various body joints such as the knee has been done before, as for example, by some type of positioning device. One type of knee positioning device is disclosed and claimed in co-pending application Ser. No. 07/979,532, filed Nov. 23, 1992, in the names of Kriedler et al. Besides manipulating or articulating two body parts in the aperture of the MRI apparatus by the above orthopedic positioning device, it is necessary to measure the intermittent or successive angles of several positions. In the above application the positioning device is connected to a digital readout type encoder device. However, for optimum results, the encoder must not interfere with the existing magnetic fields present in the MRI apparatus. Such interference might be caused here by magnetic materials in the encoder or by metallic (albeit nonmagnetic) materials such as aluminum which might produce unwanted eddy currents which affect the gradient fields; both of the foregoing tend to distort the magnetic fields (both d.c. and a.c.--RF and gradient fields) or alternatively affect the accuracy of the MRI apparatus.